If you wish to pursue practical training through an internship with a U.S. based employer you will require either an exchange visitor (J-1) or trainee (H-3) visa. Such activities cannot be conducted on a B-2 visa or visa free under the Waiver Program, even if you will receive no payment from a U.S. source.
The first step is to determine if the prospective employer is a designated exchange visitor program sponsor. Training opportunities exist in variety of occupational categories including the arts and culture, information media and communications, education, management, business, commerce and finance. If the prospective employer is a designated exchange visitor program sponsor they will issue you with a Certificate of Eligibility for Exchange Visitor Status, form DS-2019, which you will be required to furnish when applying for a J-1 visa.
If no exchange program exists, you may be eligible to obtain the requisite sponsorship for the J-1 visa through an approved work exchange program. There are a number of work exchange programs between the United Kingdom and the United States which allow participants to perform any employment which is available to them, others are restricted to a specific field. Further information is available from the individual program sponsor.
If you are unable to obtain sponsorship for an exchange visitor visa, your prospective employer will be required to file, on your behalf, a petition to accord you trainee (H-3) status. The petition, Form I-129H, is filed with the U.S. Citizenship and Immigration Services (USCIS) having jurisdiction over the area of intended employment. The petition will take several months to be processed. Please follow this link for further information.